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TOPIC 1 GEOLOGIC TIME AND GEOLOGIC PRINCIPLES - INTRODUCTION The relative geologic time scale is a product of placing rocks in their proper sequence of formation to create a composite record of geologic events. History is a sequence of events placed in proper chronological order. Placing geologic events in their proper chronological order was first done by building composite sections in one area and correlating these sections to other areas where the record could be compared and improved upon. This process has built the geologic column as we know it today. In the later half of the twentieth century, radiometric age dating techniques allowed geologists to determine absolute ages for this time scale, creating an absolute geologic time scale. Selective determination of absolute radiometric ages of important sections has allowed geologists to place ages on the geologic column, producing the geologic time scale. We will examine how relative and absolute geologic ages are determined. Later we will see how these techniques are used to study the physical and biological history of the planet. IMPORTANT PRINCIPLES The following are some of the important principles used to determine the relative ages of rock sequences. Principle of superposition: In any undisturbed succession of strata, oldest strata are at the bottom with successively younger ones above. A layer of rock is always older than the layer above it, unless they have been turned upside down. Principle of original horizontality: Particles which settle under the influence of gravity settle to more nearly horizontal layers. Generally sediments deposited on land or in the sea form nearly horizontal layers. In some cases, they are inclined, as on the sloping face of a sand dune, but parallel to the surface of deposition. In most cases, if an area has strongly inclined or dipping sedimentary rocks, they were disturbed by forces from their original horizontal attitude. Principle of original lateral continuity: Strata extend in all directions until they thin against the edges of the basins in which they have accumulated. The original continuity of strata in a basin can be broken or faulted. For example, a river can carve a canyon into a basin filled with sediment leaving once-continuous layers stranded on opposite sides of the canyon. According to the principle of lateral continuity, these opposite sides of the
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valley were once laterally continuous. Principle of cross-cutting relationships: Disturbed rocks or strata are older than those rocks which disturb them or cut through them. Liquid magma can be forced or intruded into pre-existing rocks from their sources below. After these intrusions cool, they form igneous rock that appears to cut across the other rock. Obviously the intrusive igneous rock is younger than the rock into which it cuts. Faults also cut through rocks and those rocks are older than the fault that cuts through them.The age of faults can often be
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